If Colorado seems exciting now, imagine what it must have felt like in the 1870s. We joined the Union that decade and saw the beginnings of the Colorado Silver Rush, during which about $80 million worth of silver was mined (and that's 1870s money, mind you).

That caught the attention of some money-hungry business folks, who came to Colorado in droves for our natural resources and pretty views. And with them came a stronger economy and opulent architecture.

Which leads us to today. Or, rather, October 7.

That Friday, Platinum Luxury Auctions will host an auction for Redstone Castle, located due west of Aspen. Built in 1902 for coal magnate John Cleveland Osgood, the 23,000-square-foot mansion with 42 rooms has served many purposes over the years, from Osgood's private residence to a bed-and-breakfast to a historic home now open for touristic pleasures. In honor of the Castle, we rounded up some of the spectacular former private homes, streetcar barns and dry-goods retailers that now lend their bones to a different purpose...like a sushi place.

The Redstone Inn, formerly a bachelor pad.

Courtesy Redstone Inn

1. Redstone Inn In addition to his Redstone Castle, Osgood built 84 Swiss-chalet-style homes in Redstone to house miners with families employed by his company and a twenty-room inn for the bachelors in his crew. The twenty rooms now make up the Redstone Inn.

2. Glen-Isle Though Glen-Isle has always served as a lodge, its audience has changed over the years. Built in 1901, Glen-Isle "provided a resort destination for travelers on the Colorado & Southern Railroad," according to History Colorado. Train service was discontinued in 1937, but the lodge in Bailey remained open to travelers until 2012. Like Redstone Castle, Glen-Isle is now for sale.

Hotel Teatro's restaurant, The Nickel.

Danielle Lirette

3. Hotel Teatro Built in 1911, the Tramway Building was a red-brick office building with an attached streetcar barn that served as the city's public transportation system headquarters. When streetcars became obsolete after World War II, the University of Colorado at Denver acquired the building to serve as its downtown campus. Several owners (including the Denver Center for the Performing Arts) later, the building is now home to the Hotel Teatro.

4. Cherokee Ranch & Castle Here's a word you don't hear much of nowadays: homestead. But that's what Cherokee Ranch & Castle was back in the late 1890s — two separate family homesteads. After moving through a few owners, the property fell to Charles Alfred Johnson, who built the spectacular Charlford Castle in the 1920s as his summer residence before moving in full-time. The property is now home to a wildlife sanctuary, and the castle hosts weddings and other events.